Lignin is the second most abundant renewable natural biopolymer on the planet next to cellulose and it is massively generated as a by-product from papermaking and emerging cellulosic ethanol industries. Annually, more than 50 million tons of lignins are produced but only about 2% of the lignins are used in value added applications, including the isolation of chemicals, stabilizing agents and concrete additives, while the rest is used as low grade burning fuel. As fossil fuels are being consumed and their negative influences on environment are increasing, lots of efforts have been put into developing value added materials using lignin to substitute fossil fuel based products due to its abundant availability and renewable resources. On the other hand, the waste lignin provides various advantages, such as adequate reactive groups that can easily be functionalized, high carbon content, low density, being biodegradable and environmentally friendly, antioxidant, antimicrobial, and stabilizer properties, tunable rheological and viscoelastic properties, tailored ability for chemical transformations, continuous production during paper making, high volumes, etc., making it a potential candidate to be used in diverse industrial applications.
Lignin is a randomly cross-linked network biopolymer arising from enzymatic dehydrogenative polymerization of hydroxylated and methoxylated phenylpropane unit, and recently a growing interest has been paid on utilizing lignin's hydrophobic polyol structure to develop novel lignin-based functional materials. However, very few studies have been done on lignin-based hydrogels.
Hydrogel is a class of polymer networks with hydrophilic groups, which enable the absorption of water while remaining resistant to dissolution as the physical or chemical crosslinks formed among their molecules. With the special structure, hydrogels possess various advantages, such as high water content, easy operation, biocompatibility and mechanical properties, and they have been explored for applications in biological medicine, genetic delivery, tissue engineering, and biomedical materials.
Compared to non-injectable chemically crosslinked hydrogels with non-reversible crosslinked structures, physical hydrogels are preferred for biomedical application as they are able to be injected from syringes and allowed to set in the body. Supramolecular hydrogels are physical networks self-assembled by biocompatible gelators with macromolecular or low-molecular-weight molecules via noncovalent interactions, including hydrogen bonding, hydrophobic interactions, host-guest recognition, and crystallization.
However, conventional hydrogels may not be sufficiently strong for the applications that they are used in, may not be able to self-heal when damaged, or may be toxic to a living human or animal body.
There is a need to provide a hydrogel that overcomes, or at least ameliorates, one or more of the disadvantages described above. There is a need to provide a copolymer that can, in one application, be used to form the hydrogel.